For a startup from standstill with a multi-stage stepped transmission, in principle, several gears can be considered for the startup gear. One such startup situation occurs in particular in the case of a startup on a plane and on an incline. With the startup, the engine torque that can be generated by the drive engine and transmitted to the friction clutch as the startup torque must be sufficiently high in order to compensate for the stationary drive resistance of the motor vehicle, which is formed in this situation by the rolling resistance and incline resistance, given the overall transmission ratio determined by the respective startup gear and the efficiency of the drive train, and in addition, to deliver excess torque for startup acceleration of the motor vehicle.
In the process, it must be considered that active output drive-side power take-offs, that is, power take-offs disposed at the transmission and/or the axle transmission, reduce the engine torque that can be used for startup, which can be considered as a fictional additional resistance for the determination of the startup gear. In contrast, auxiliary consumers driven directly by the drive engine, such as an electric generator, a servo pump of a servo steering, and an air conditioning compressor of an air conditioning system, as well as active drive-side power take-offs, that is, power take-offs disposed directly at the drive engine, reduce, already at the source of the rotational energy, the engine torque that can be delivered by the drive engine to the friction clutch and available for startup.
Furthermore, the startup acceleration should correspond to the respective power request by the driver, which is given by the gas pedal deflection or the gas pedal position respectively, increasing with increasing gas pedal deflection and decreasing with increasing road incline. With increasing gas pedal deflection at a constant road incline, the driver accordingly expects faster startup acceleration, whereas in contrast with an increasing road incline with a constant gas pedal position the driver expects slower startup acceleration.
A determination of the startup gear depending only on the startup situation typically occurs using the characteristic curves or characteristic maps, which are modified to the respective vehicle configuration using complex application methods, and which contain at least the vehicle mass, the roadway incline and the gas pedal position as parameters.
For a startup from standstill the friction clutch can be a passive engageable single or multi-disc dry clutch or an active engageable multi-disc clutch, for bridging the speed difference between the engine speed and the transmission input speed and the transmission input shaft in slipping operation, until the motor vehicle has accelerated to the extent that synchronous running is attained at the input and output sides of the friction clutch so that the clutch can be completely engaged.
The startup-dependent slipping operation represents a high mechanical and thermal load for the friction clutch that increases with the value of the startup torque, the value of the slip speed and the duration of the slipping operation, and which forms an essential parameter for determining the startup gear.
If the startup gear is set too low, fast startup acceleration and a correspondingly shorter slipping operation of the friction clutch is possible. Due to the high transmission ratio of the startup gear, noise develops, and due to the high startup speed, the fuel consumption of the internal combustion engine is unfavorably high.
In addition, due to the fast startup acceleration a shift speed is attained relatively quickly and a shift is triggered to a higher gear. This is considered uncomfortable and particularly at high drive resistance, for instance on a steep incline or on difficult terrain, can lead to a strong delay of the vehicle during the shift-dependent interruption of the tractive force and consequently to an interruption of the startup.
If in contrast, the startup gear is too high, the slipping speed is relatively high at the friction clutch due to the low transmission ratio of the startup gear. Due to the slow startup acceleration, the duration of the slipping operation can be so long that the friction clutch is thermally overloaded.
Therefore, the general aim is to perform a startup of a motor vehicle in the highest possible gear, however without mechanically and thermally overloading the friction clutch in the process. Thus, methods for determining a startup gear are known from the documents DE 198 39 837 A1 and U.S. Pat. No. 6,953,410 B2, with which the highest possible startup gear is determined from the present drive resistance of the motor vehicle and the available engine torque of the drive train so that the expected duration of slipping of the friction clutch during the startup and/or the thermal energy created in friction clutch in slipping operation do not exceed predetermined limit values.
The document U.S. Pat. No. 7,220,215 B2 describes a commercial vehicle with a control device with which the highest possible startup gear is determined so the maximal engine torque that can be generated by the drive engine at idle speed is sufficient for the startup, and the thermal energy created in the process in the friction clutch does not exceed a predetermined limit value.
In the case of commercial vehicles, the drive engines are usually designed as turbo-charged diesel engines, which have a specific load build-up characteristic. According to the document DE 10 2008 054 802 A1, which was previously unpublished, and which discloses a method for controlling an automatic stepped transmission depending on the dynamic operating characteristics of a turbo-charged internal combustion engine, a turbo-charged internal combustion engine can spontaneously, that is with high torque gradients, only reach an intake torque lying below the full load torque.
A further increase of the engine torque is briefly possible, although with low torque gradients, only above a boost threshold speed, after which the turbo-charger creates a significant increase of the charge pressure and thus the engine torque. Thus, aside from the idle speed, cut-off speed and the full load torque characteristic curve, the dynamic behavior of a turbo-charged internal combustion engine is also determined by the boost threshold speed and the intake torque characteristic curve as well as by the present torque gradients, at least in certain regions.
Therefore, the dynamic operating properties of a drive engine built as a turbo-charged internal combustion engine are also significant for determining a startup gear, because starting from the idle speed only the intake torque is spontaneously built up and usable as the startup torque. If the intake torque is not sufficient as startup torque, the engine speed must be increased above the boost threshold speed, in order to be able to increase the engine torque above the intake torque by increasing the charge pressure. In this case however, due to the hereby increased slipping speed and the slowdown of the torque buildup, the mechanical and thermal load of the friction clutch increases significantly.
With the previously known methods for determining a startup gear, the present load state of the friction clutch, repeated startups without significant cooling of the friction clutch in between, and the dynamic operating properties of the drive engine were not considered, or not sufficiently considered. This can have the consequence that the friction clutch, despite nominally maintaining the intended load limit, is mechanically and/or thermally overloaded, and consequently does not attain an intended service life or is destroyed during a startup procedure.